Question

What is the difference between the following 2 queries? (They both produce the same results)

select *
from (
  select * from (
           select *
           from phppos_items
           where name like 'AB10LA2%' and deleted = 0
           order by `name` limit 16
           ) t
  union
  select * from (
           select *
           from phppos_items
           where item_number like 'AB10LA2%' and deleted = 0
           order by `name` limit 16
           ) t
  union
  select * from (
           select *
           from phppos_items
           where category like 'AB10LA2%' and deleted = 0
           order by `name` limit 16
           ) t
  ) as top_rows
order by `name` limit 16

vs

select *
from (
           (select *
           from phppos_items
           where name like 'AB10LA2%' and deleted = 0
           order by `name` limit 16)
  union
           (select *
           from phppos_items
           where item_number like 'AB10LA2%' and deleted = 0
           order by `name` limit 16)
  union
           (select *
           from phppos_items
           where category like 'AB10LA2%' and deleted = 0
           order by `name` limit 16)
  ) as top_rows
order by `name` limit 16

Answer 1

The first version has another set of temporary tables, which is useless and a waste of resources in this specific case.
All the following will produce same result:

SELECT * FROM T1;

SELECT * FROM (SELECT * FROM T1);


SELECT * FROM (SELECT * FROM (SELECT * FROM T1));

...
...

to infinity 🙂

And in your very specific case this will be enough:

select *
           from phppos_items
           where 
                 (name like 'AB10LA2%' OR item_number like 'AB10LA2%' OR category like 'AB10LA2%')
           and deleted = 0
           order by `name` limit 16